[en] Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB₂ are presented. The E_2g phonons, which involve in-plane Bdisplacements, couple strongly to the p_x,y electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting λ in the clean limit is calculated to be significantly larger. The E_2g phonons are strongly anharmonic, and the nonlinear contribution to the coupling between the E_2g modes and the p_x,y bands is significant

[en] Interface structures of the Fe/ZrO₂-system are presented. We investigated several possible orientations and configurations by means of density functional theory. Special emphasis was put on mechanical stability, termination of the different interface-structures and magnetic effects. The energetic comparison gives a view on the order of occurrence. As an addition, some experimental data is presented.

No abstract available

[en] This review is devoted to the current theoretical understanding of MgB₂, mainly focusing of the effect of impurities and doping on the physical properties of the material. We compare the theoretical predictions with experimental results in order to suggest an explanation for the behavior of the superconducting gaps and critical temperature with doping

[en] The valence shell of boron contains less electrons than available orbitals, which results in very complex bulk structures, primarily consisting of B₁₂ icosahedra. But boron is relatively little studied and many properties such as the phase diagram, the ground state structure, and its high-pressure behavior are unknown. Recently two discoveries put elemental boron into the scientific focus. One is that it becomes superconducting under high pressure. The other one is that small clusters form two-dimensional sheet-like structures, as suggested by theoretical studies of Boustani and Quandt in the mid-1990s. The high-pressure superconductivity is not fully understood so far, because the corresponding crystal structures are still under debate. Hence before solving the problem of understanding the coupling mechanism for superconductivity one has to solve first the problem of determining the crystal structure. The existence of sheet-like clusters poses the question if boron sheets, similar to graphene, or layered bulk phases, similar to graphite, may also exist for boron. Therefore our study tries to approach the following questions: What do such layered bulk structures look like? What are their stabilities in comparison with other bulk phases? Are they dynamically stable and if yes, are they responsible for the high-pressure superconductivity of elemental boron?

[en] Boron has recently received a lot of interest in the search of new electron-phonon superconductors. Elemental boron has been shown to undergo a superconducting transition at high pressures. However, no experimental data about the actual crystal structure of the superconducting phase are available. In this work we examine the relative stability of different possible high-pressure phases of boron and calculate the phonon dispersions at the experimental pressures at which superconductivity is observed. The total-energy calculations show that the - Ga phase of boron, which gives a metal with a low DOS, remains the most energetically favoured one up to very high pressures. Our linear-response results indicate further, that several of the in the literature proposed structures are actually unstable at high pressure

[en] We have used first-principles calculations based on density-functional theory to investigate the ¹¹⁹Sn Moessbauer spectrum of a-Ge_0.99xSn_0.01xSe_1-x and a-Ge_0.99xSn_0.01xS_1-x. Using calculated electric field gradients and contact charge densities, we compute Moessbauer isomer shifts and quadrupole splittings for a number of cluster models incorporating proposed environments for Sn atoms in the glasses. The calculated parameters are in excellent agreement with experimental values for tetrahedrally coordinated Sn atoms and for ionic, threefold-coordinated Sn atoms. Parameters computed for Sn atoms in ethanelike environments, however, do not match experimental values attributed to these sites. We also compute site energies to determine the most energetically favorable sites for Sn atoms in these systems. For the Ge-S system, we find the threefold environments to be favored, while for Ge-Se, the threefold and tetrahedral environments are essentially degenerate

[en] We present the results of a study of the high pressure phase diagram of elemental boron, using full-potential density functional calculations. We show that at high pressures (P > 100 GPa) boron crystallizes in quasi-layered bulk phases, characterized by in-plane multicenter bonds and out-of-plane unidimensional sigma bonds. These structures are all metallic, in contrast to the low-pressure icosahedral ones, which are semiconducting. We show that the structure and bonding of layered bulk phases can be easily described in terms of single puckered boron sheets. Our results bridge the gap between boron nanostructures and bulk phases.

[en] It has recently been shown using high-resolution transmission electron microscopy that there may exist an hetero-epitaxial interface between austenitic steel and cubic ZrO₂ for a certain crystallographic orientation between these two materials. Here we discuss possible bonding at that interface by means of density-functional-theory calculations. In particular, we use the electron localization function as a measure for bonding across the interface. We find that there should be no strong interface in the case of pure Fe and ZrO₂. However, Si or C atoms at the interface layer appear to be able to bridge the interface providing possible covalent bonds. In contrast, metal atoms seem not to be able to provide sufficient changes to connect across the interface. The findings are rationalized in terms of ionic radius and electro-negativity of the atom. (paper)

[en] The optical constants together with the magneto-optical Voigt constants of several phthalocyanine (Pc) and methoxy functionalized tetraphenylporphyrin (TMPP) thin films prepared on silicon substrates are presented. The materials investigated are MePc with Me = Fe, Co, Ni, Cu, Zn and MeTMPP with Me = Cu, Ni. We also compared our results to the metal-free H₂Pc, H₂TPP and H₂TMPP. The experimental results will be supported by electronic structure calculations based on density functional theory (DFT) and interpreted using the perimeter model initially proposed by Platt. The model allows for qualitative understanding of the forbidden character of transitions in planar, aromatic molecules, and is able to qualify differences between Pc and TMPP type materials. (paper)